Add xcode version information

What does this MR do?

Compile data from Mactracker & Wikipedia to inform which versions of Xcode to support. This is based on @dehesselle's thread here: gitlab-org/gitlab-runner#6564 (comment 462485395)

Questions:

• Do we want to add xcode 8 & 9 based on these tables? Are we ok to support that many versions?
• Right now this is only an arbitrary choice. Should we have a rule like "last X major versions" or "last X years of apple support for devices"?

Why was this MR needed?

What's the best way to test this MR?

What are the relevant issue numbers?

#47 (closed)